document that echo can't be given a NULL

[Dan Jacobson]

No way to hand echo or /bin/echo a NULL.
$ set a $'\x00' b
$ echo $#
3
$ echo A$'\x01'B|wc -c
4
$ echo A$'\x00'B|wc -c
3
$ echo -n `echo -ne \\\\000`|wc -c
0
BASH_VERSION='3.1.0(1)-release'

At least the echo docs should say so.